1. Technical Field of the Invention
This invention relates to a thermal printer comprising a large number of heating elements made of resistors, etc., and an electric motor for transporting recording paper little by little at the recording time.
2. Description of the Related Art
A thermal printer comprising a large number of heating elements divided into groups for activating the groups at different timings (asynchronously) in a time-divisional manner to reduce current capacity of a power supply, etc., is available.
On the other hand, for example, a thermal printer for recording while transporting recording paper one line at a time by a stepping motor may record in both stopping and moving periods of recording paper to raise the recording speed.
In a thermal printer in a related art for activating heating elements in a time-divisional manner and recording also in the moving period of recording paper, all groups are activated only through one order every turn-on period of a driving pulse of a stepping motor. For example, to divide a large number of heating elements of a thermal print head into a left group and a right group relative to the center of the longitudinal direction of the thermal print head and activate the left and right groups separately (each once), a first strobe signal STB1 is turned on once for activating the left heating elements and then a second strobe signal STB2 is turned on once for activating the right heating elements every turn-on period of a driving pulse of a stepping motor, as shown in FIG. 11, or a first strobe signal STB1 is turned on more than once for activating the left heating elements and then a second strobe signal STB2 is turned on more than once for activating the right heating elements every turn-on period of a driving pulse, as shown in FIG. 12.
However, with the thermal printer in the related art, there occurs unevenness in density on a recorded image because the rotation operation of the stepping motor is unstable; high-quality recording cannot be accomplished.
That is, the rotation speed of the stepping motor is not always constant during the turn-on period of a driving pulse and varies considerably. Particularly, inexpensive stepping motors must often be adopted to reduce the manufacturing costs of the thermal printers. In such a case, the rotation speed of the stepping motor varies hard and even the time between the rising edge of a driving pulse and the rotation start of the actual rotation shaft of the stepping motor is not constant, causing large variations to occur. However, if all groups are activated only through one order every turn-on period of a driving pulse of the stepping motor as in the thermal printer in the related art, the rotation speed of the stepping motor during the activating period varies remarkably from one group to another; for example, the left half portion of an image recorded on recording paper becomes remarkably darker than the right half portion.